The present invention relates to the field of measuring the optical properties of materials, in particular the determination of the optical rotatory dispersion, or circular birefringence of a sample.
Many biopolymers and macromolecules posses chirality and exhibit optical activity when light is transmitted through the material. Optical activity is defined as the property in which a material absorbs incident radiation and/or changes its polarization state. A material that changes the polarization state of the incident radiation exhibits circular birefringence. A material that absorbs incident radiation depending on the polarization state of the incident radiation exhibits circular dichroism. The optical activity of a substance may be modeled by assigning distinct indices of refraction for left circularly polarized (xe2x80x9cLCPxe2x80x9d) light, xcex7l, and for right circularly polarized (xe2x80x9cRCPxe2x80x9d) light, xcex7r, where xcex7l and xcex7r are both complex. The difference between the real part of the complex indices of refraction for the LCP and RCP light, xcex94n=(nlxe2x88x92nr)=Re(xcex7l)xe2x88x92Re(xcex7r), describes the circular birefringence of the material. The difference between the imaginary part of the complex indices of refraction for the LCP and RCP light, xcex94k=(klxe2x88x92kr)=Im(xcex7l)xe2x88x92Im(xcex7r), describes the circular dichroism of the material.
Light that is transmitted through a circular birefringent material will exhibit a phase angle rotation, xcex1, given by the equation
xcex1=(xcfx80d/xcexo)(nlxe2x88x92nr)xe2x80x83xe2x80x83(1) 
where d is the thickness of the sample and xcexo is the wavelength of the incident radiation. The difference in the indices of refraction, (nlxe2x88x92nr), is referred to as the circular birefringence or rotatory dispersion of the sample
In order to measure the phase angle rotation of a sample, traditional systems transmit light of a single known polarization state through the material and measure the polarization state of the transmitted light. The polarization state of the incident light is usually selected by passing the incident light through a linear polarizer (xe2x80x9cgenerating polarizerxe2x80x9d). The rotation of the linear polarized light about the optical axis defined by the light beam is measured by passing the light beam exiting the sample through a second linear polarizer (xe2x80x9cmeasurement polarizerxe2x80x9d) and measuring the transmitted light intensity exiting the measurement polarizer. The measurement polarizer is rotated about the optical axis until the transmitted light intensity is a maximum. The angle of the measurement polarizer with respect to the generating polarizer at maximum transmitted light intensity represents the phase angle rotation of the sample.
The advantage of the traditional systems is that the measurement of the rotation of the measurement polarizer to the generating polarizer is a direct measurement of the circular birefringence of the sample. The disadvantage of the traditional systems is that the resolution and accuracy of the rotation measurement is limited by the polarizers and the mechanical limitations of the polarizer mounting stages. The polarizers are susceptible to thermal fluctuations of the environment that require the user to place the system in a controlled environment and reduce the measurement time as much as possible. This usually requires placing the sample in an enclosed chamber with a closely monitored environment. Placing the sample in a chamber further restricts the type of operations or measurements that can be performed on the sample. As a result of these restrictions, phase measurement systems in current use have resolutions between 1.0-0.1xc2x0.
Instead of passing light of a single polarized state through the sample, U.S. Pat. No. 5,896,198 issued on Apr. 20, 1999 to Chou et al. uses an optical heterodyne beam consisting of two linearly polarized waves wherein the polarization planes of the two beam are orthogonal to each other. The beams exiting the sample are passed through an analyzing polarizer. The intensity of the beam is measured by a photo-detector and if the rotation of the beam is small, the measured intensity will be proportional to the rotation of the beam. The analyzing polarizer is rotated in a calibration setup that maximizes the transmitted intensity of the beam but is not used to measure the rotation of the beam. The orientation of the polarizer, however, is still important. The rotation of the beam will be proportional to the circular birefringence of the sample only if the two linearly polarized waves remain orthogonal after passing through the sample. In addition, since the rotation is proportional to the transmitted intensity of the beam, small rotations will produce lower intensities such that noise in the detection system or extraneous light sources will limit the minimum resolution of the system.
Therefore, there remains a need for a measuring device that provides for real-time, or instantaneous, measurement of the sample to resolutions of  less than 0.1xc2x0 while allowing easy access to the sample during the measuring process.
A light beam comprised of RCP and LCP waves of different frequencies is presented by a beat frequency and a beam phase. The beam phase contains information on the difference between the RCP phase and LCP phase. As the light beam passes through an optically active material, the RCP phase and/or the LCP phase will change. The change in either or both of the RCP phase or LCP phase is contained in the beam phase. Two measurements are made, one with a blank sample and one with the sample. The blank and sample measurements remove any path length or environmental effects from the beam phase. In order to remove any temporal drift effects, each measurement is adjusted by a reference measurement taken at the same time as the blank and sample measurements.
One aspect of the present invention is directed to an apparatus for measuring the circular birefringence of a sample comprising: a light beam generator generating a light beam having a right circularly polarized (RCP) wave characterized by a first frequency and a left circularly polarized (LCP) wave characterized by a second frequency; a beam splitter positioned to receive the light beam from the beam generator and produce a measurement beam and a reference beam; a reference polarizer positioned to receive the reference beam and produce a reference heterodyne wave, the reference heterodyne wave characterized by a reference phase, the reference phase representing the difference between the RCP wave and the LCP wave of the reference beam; a reference detector positioned to receive the reference heterodyne wave and generate a reference signal; a measurement polarizer positioned to receive the measurement beam exiting the sample and produce a measurement heterodyne wave, the measurement heterodyne wave characterized by a measurement phase, the measurement phase representing the difference between the RCP wave and the LCP wave of the measurement beam; a sample detector positioned to receive the measurement heterodyne wave and generate a measurement signal; a gain/phase meter connected to the reference detector and sample detector and generating an output signal characterized by a phase difference equal to the difference between the measurement phase and the reference phase; and a processor connected to the gain/phase meter, the processor calculating the circular birefringence of the sample based, in part, on the output signal of the gain/phase meter.
Another aspect of the present invention is directed to a method for determining the circular birefringence of a sample comprising the steps of: generating a coherent light beam having a left circularly polarized (LCP) wave and a right circularly polarized (RCP) wave, the light beam characterized by a phase; splitting the light beam into a measurement beam and a reference beam; passing the measurement beam through a blank sample; measuring a first phase difference between the measurement beam and reference beam; replacing the blank with the sample and passing the measurement beam through the sample; measuring a second phase difference between the measurement beam and reference beam; and determining the circular birefringence of the sample base, in part, on the first and second phase difference.